@article {23, title = {Mitochondrial genomes reveal slow rates of molecular evolution and the timing of speciation in beavers (Castor), one of the largest rodent species.}, journal = {PLoS One}, volume = {6}, year = {2011}, month = {2011 Jan 28}, pages = {e14622}, abstract = {

BACKGROUND: Beavers are one of the largest and ecologically most distinct rodent species. Little is known about their evolution and even their closest phylogenetic relatives have not yet been identified with certainty. Similarly, little is known about the timing of divergence events within the genus Castor.

METHODOLOGY/PRINCIPAL FINDINGS: We sequenced complete mitochondrial genomes from both extant beaver species and used these sequences to place beavers in the phylogenetic tree of rodents and date their divergence from other rodents as well as the divergence events within the genus Castor. Our analyses support the phylogenetic position of beavers as a sister lineage to the scaly tailed squirrel Anomalurus within the mouse related clade. Molecular dating places the divergence time of the lineages leading to beavers and Anomalurus as early as around 54 million years ago (mya). The living beaver species, Castor canadensis from North America and Castor fiber from Eurasia, although similar in appearance, appear to have diverged from a common ancestor more than seven mya. This result is consistent with the hypothesis that a migration of Castor from Eurasia to North America as early as 7.5 mya could have initiated their speciation. We date the common ancestor of the extant Eurasian beaver relict populations to around 210,000 years ago, much earlier than previously thought. Finally, the substitution rate of Castor mitochondrial DNA is considerably lower than that of other rodents. We found evidence that this is correlated with the longer life span of beavers compared to other rodents.

CONCLUSIONS/SIGNIFICANCE: A phylogenetic analysis of mitochondrial genome sequences suggests a sister-group relationship between Castor and Anomalurus, and allows molecular dating of species divergence in congruence with paleontological data. The implementation of a relaxed molecular clock enabled us to estimate mitochondrial substitution rates and to evaluate the effect of life history traits on it.

}, keywords = {Animals, Evolution, Molecular, Genetic Speciation, Genome, Mitochondrial, Kinetics, Phylogeny, Rodentia}, issn = {1932-6203}, doi = {10.1371/journal.pone.0014622}, author = {Horn, Susanne and Durka, Walter and Wolf, Ronny and Ermala, Aslak and Stubbe, Annegret and Stubbe, Michael and Hofreiter, Michael} } @article {25, title = {Re-evaluation of Sinocastor (Rodentia: Castoridae) with implications on the origin of modern beavers.}, journal = {PLoS One}, volume = {5}, year = {2010}, month = {2010 Nov 15}, pages = {e13990}, abstract = {

The extant beaver, Castor, has played an important role shaping landscapes and ecosystems in Eurasia and North America, yet the origins and early evolution of this lineage remain poorly understood. Here we use a geometric morphometric approach to help re-evaluate the phylogenetic affinities of a fossil skull from the Late Miocene of China. This specimen was originally considered Sinocastor, and later transferred to Castor. The aim of this study was to determine whether this form is an early member of Castor, or if it represents a lineage outside of Castor. The specimen was compared to 38 specimens of modern Castor (both C. canadensis and C. fiber) as well as fossil specimens of C. fiber (Pleistocene), C. californicus (Pliocene) and the early castorids Steneofiber eseri (early Miocene). The results show that the specimen falls outside the Castor morphospace and that compared to Castor, Sinocastor possesses a: 1) narrower post-orbital constriction, 2) anteroposteriorly shortened basioccipital depression, 3) shortened incisive foramen, 4) more posteriorly located palatine foramen, 5) longer rostrum, and 6) longer braincase. Also the specimen shows a much shallower basiocciptal depression than what is seen in living Castor, as well as prominently rooted molars. We conclude that Sinocastor is a valid genus. Given the prevalence of apparently primitive traits, Sinocastor might be a near relative of the lineage that gave rise to Castor, implying a possible Asiatic origin for Castor.

}, keywords = {Animals, China, Female, Fossils, Geography, Male, North America, Phylogeny, Rodentia, Skull, Species Specificity}, issn = {1932-6203}, doi = {10.1371/journal.pone.0013990}, author = {Rybczynski, Natalia and Ross, Elizabeth M and Samuels, Joshua X and Korth, William W} } @article {28, title = {Comparative morphological study on the stereo-structure of the lingual papillae and their connective tissue cores of the American beaver (Castor canadensis).}, journal = {Okajimas Folia Anat Jpn}, volume = {82}, year = {2006}, month = {2006 Feb}, pages = {127-37}, abstract = {

The lingual papillae and the connective tissue cores (CTC) of the American beaver were examined by light and scanning electron microscopy. The tongue of American beaver was about 9 cm in length, 3.5 cm in width, and has a lingual prominence. Four types of papillae (filiform, fungiform, vallate and foliate papillae) were observed. The filiform papillae can be classified into three types (filiform, large filiform and dorm-like papillae). Filiform papillae distributed on the anterior tongue and posterior of the lingual prominence consisted of a posterior thick main process and several small accessory processes. After removal of the epithelium, the CTCs of the filiform papillae had U-shaped, horseshoe-like primary cores with 10-15 rod-shaped small accessory cores. Large filiform papillae were distributed at the anterior margin of the lingual prominence. Dome-like papillae were distributed at the top of lingual prominence. Fungiform papillae were observed two types. Fungiform papillae, which were distributed at the anterior tongue, were round shaped. Fungiform papillae of the posterior of the lingual prominence were large and surrounded with a papillary groove. At the posterior of the tongue, three vallate papillae were arranged in a triangular pattern. Foliate papillae were on 22 to 25 parallel ridges and grooves.

}, keywords = {Animals, Connective Tissue, Epithelium, Feeding Behavior, Female, Male, Microscopy, Microscopy, Electron, Scanning, Rodentia, Tongue}, issn = {0030-154X}, doi = {10.2535/ofaj.82.127}, author = {Shindo, Junji and Yoshimura, Ken and Kobayashi, Kan} }